Nitrogen, biochar, and mycorrhizae: Alteration of the symbiosis and oxidation of the char surface

In some cases amending soil with biochar improves fertility, although the exact mechanisms through which biochar alters soil processes are not well understood. In other cases, however, biochar amendment can have no effect on plant growth, or can have negative effects. When crop benefits occur, simultaneous amendment with biochar and mineral nutrients causes results that are not additive, suggesting that biochar may be capable of improving the efficiency of nutrient uptake by plants, but the mechanisms of this synergy remain unknown. One possible mechanism that has not been fully explored is alterations to the plant-mycorrhizal fungus mutualism, a relationship that occurs in most land plants. In a 4 week greenhouse experiment, we investigated possible effects of the presence of biochar, mycorrhizal fungi, and nitrogen fertilizer on sorghum seedling growth. Results indicated that the combined treatment of biochar, mycorrhizal fungi, and high nitrogen decreased aboveground plant biomass by 42% relative to the mycorrhizae and high nitrogen treatment, while simultaneously promoting mycorrhizal root colonization. This is evidence for an induced parasitism of the mycorrhizal fungus in the presence of nitrogen and biochar within the 4 week timescale of our experiments. Using x-ray photoelectron spectroscopy, we found evidence of increased surface oxidation on biochar particles over the 4 weeks of our trial, consistent with sorption of labile, plant derived dissolved organic matter or char oxidation, either via biotic or abiotic processes. Biochar in soils with mycorrhizae but without sufficient nitrogen showed more surface oxidation than other treatment combinations, and showed a significantly greater fraction of surface carbon present in carbonyl (ﾖCdouble bond; length as m-dashO) functionalities. Our results suggest that soil nitrogen acts as a switch controlling the ability of char to influence the mycorrhizal symbiosis and, in turn, the degree to which the fungi oxidize the char surface